Exhaust Systems 101

HammerHead · Feb 27, 2015

I see a lot of exhaust questions on this forum and wanted to share what I learned. These articles cover back pressure, Scavenging, pipe size, pipe length and pressure waves all that goes into the complexities of the exhaust system. This is hours of reading but if you want the best out of your exhaust system this is a good start. And hopefully you will make the right exhaust decision the first time and save some $$$.

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Page 7

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kontrolled · Mar 4, 2015

bump for awarness and general good knowledge.

jsteinm1 · Mar 5, 2015

Whoa, thanks for posting up all the info! Exhaust systems are definitely complex. I got into engine design a little bit during my mech engineer undergrad work but I definitely don't know/remember all the concepts.

HammerHead · Mar 5, 2015

jsteinm1 said:
Whoa, thanks for posting up all the info! Exhaust systems are definitely complex. I got into engine design a little bit during my mech engineer undergrad work but I definitely don't know/remember all the concepts.

kontrolled said:
bump for awarness and general good knowledge.

Thanks guys. I cut into my exhaust system a few months ago and lost some low end and was determined to understand why. After hours of research and hours of reading I was amazed at how complex exhaust systems are. What I learned was priceless and if I had know what I know now I could have saved myself some money.
So many threads and questions about exhaust systems on the forum I wanted to consolidate my findings and hopefully help a fellow member make a good decision the first time and save some $$$ :favorites13:

14hemiexpress · Mar 5, 2015

I love the garden hose analogy because is something we all know and have done it puts velocity into perspective on how smaller pipes can acually remove exhuast gasses more effectively.

I use to be one of the guys that used the term back pressure and was called out on it and looked into it more and realised I was WR wr wr wro wron wrong... Damn that's hard to say! It's one of things that there is a lot more to it than meets the eye.

Lots of good Info for anyone looking into exhaust systems. Thank you for taking the time to post it up!

CMil527 · Mar 5, 2015

HammerHead said:
Thanks guys. I cut into my exhaust system a few months ago and lost some low end and was determined to understand why. After hours of research and hours of reading I was amazed at how complex exhaust systems are. What I learned was priceless and if I had know what I know now I could have saved myself some money.
So many threads and questions about exhaust systems on the forum I wanted to consolidate my findings and hopefully help a fellow member make a good decision the first time and save some $$$

So that being said Hammer what do you or would you run on your rig?

sicmadek · Mar 5, 2015

14hemiexpress said:
I love the garden hose analogy because is something we all know and have done it puts velocity into perspective on how smaller pipes can acually remove exhuast gasses more effectively.

I use to be one of the guys that used the term back pressure and was called out on it and looked into it more and realised I was WR wr wr wro wron wrong... Damn that's hard to say! It's one of things that there is a lot more to it than meets the eye.

Lots of good Info for anyone looking into exhaust systems. Thank you for taking the time to post it up!

Go fill a bucket up using your finger on the hose then fill it with no retriction. Tell me which one fills faster. Restriction causes the water to shoot farther but can have a less flow rate.

Sillygoose · Mar 5, 2015

gunner1374 · Mar 5, 2015

I too lost a little low end with my dual exhaust set up, but I feel that it now hooks easier and the power up top makes up for any loss on the bottom. Lots of low end tq is good for smokey burnouts or for vehicles with posi and wide slicks, but, IMO, not much good on a heavy truck on stock-ish tires trying to get out of the hole. I much prefer being able to hook a little better, having the power to spin for a sec on the 1-2 shift at 55-60 mph, and pulls like a beast in the upper RPMs. Also, an upgraded torque converter gets you over the low end anyway so who cares? Very good info though. Thanks for taking the time to help inform others. That's what makes this site great!

HammerHead · Mar 5, 2015

CMil527 said:
So that being said Hammer what do you or would you run on your rig?

That's a hard question for me to answer at this point because I'm not sure how far I'm going to modify my truck. I got what I lost back with a custom tune so I'm just going to leave it for now. I want to get some track time in before I decide.

14hemiexpress · Mar 5, 2015

sicmadek said:
Go fill a bucket up using your finger on the hose then fill it with no retriction. Tell me which one fills faster. Restriction causes the water to shoot farther but can have a less flow rate.

Couple things 1 it can slow down just depends on how hard you hold your thumb over it. Second that speed creates a vacume as a engine doesn't produce a solid stream of exhaust it's pulses and the first one can actually pull the second out so on and so forth "it's called scavenging" 3rd smaller isn't always better there is a point were too small will cause problems there is a sweet spot depending on what rpm your trying to produce power but it's normally smaller than most people think it will be.

Hemi450hp · Mar 5, 2015

Building a proper exhaust is all relative to how you set the rest of the truck up. From all our testing at the track and dyno, we have always shown to get better peak hp numbers and better track times with an open 3" single exhaust on N/A Hemi trucks.

Our shop truck is running 1 7/8" headers with no cats, and no muffler or piping behind the Y-pipe. It doesn't get much more open than that, but when paired up with 4.10s and a 3400 stall converter, it still comes off the line hard with consistent mid 1.6 60' times.

Modifying a vehicle is a package deal, so you really want to have a goal in mind before starting any mods. I have learned this the hard way on previous vehicles and wasted a lot of money buying parts twice.

Hemi8073 · Mar 5, 2015

Is like to see you run torque tubes matt, about 18-20" long and see how it affects you time/power on the dyno and at the track

Hemi450hp · Mar 5, 2015

When I decide to finally trailer the truck, I will switch to tq tubes. Its already enough of a hassle to swap wheels, remove muffler, and install track intake when I get there. The current setup is working well, but I do think it would pick up a little top end power with the tubes.

Hemi8073 · Mar 5, 2015

I agree, it's the only reason I haven't done it yet, I just ordered the stuff to build them and will be running them at mopars at motorplex. Gonna go with dual 3" tq tubes about 20" or so, haven't decided on the length yet. I think it will pick up top end as well

HammerHead · Mar 5, 2015

14hemiexpress said:
I love the garden hose analogy because is something we all know and have done it puts velocity into perspective on how smaller pipes can acually remove exhuast gasses more effectively.

I use to be one of the guys that used the term back pressure and was called out on it and looked into it more and realised I was WR wr wr wro wron wrong... Damn that's hard to say! It's one of things that there is a lot more to it than meets the eye.

Lots of good Info for anyone looking into exhaust systems. Thank you for taking the time to post it up!

Yes sir, I Mis-understood back pressure of scavenging also. I was getting ready to cut into my exhaust system for the second time but figured I would do some research first. And I am very glad I did.

gunner1374 said:
I too lost a little low end with my dual exhaust set up, but I feel that it now hooks easier and the power up top makes up for any loss on the bottom. Lots of low end tq is good for smokey burnouts or for vehicles with posi and wide slicks, but, IMO, not much good on a heavy truck on stock-ish tires trying to get out of the hole. I much prefer being able to hook a little better, having the power to spin for a sec on the 1-2 shift at 55-60 mph, and pulls like a beast in the upper RPMs. Also, an upgraded torque converter gets you over the low end anyway so who cares? Very good info though. Thanks for taking the time to help inform others. That's what makes this site great!

+1...

Hemi450hp said:
Building a proper exhaust is all relative to how you set the rest of the truck up. From all our testing at the track and dyno, we have always shown to get better peak hp numbers and better track times with an open 3" single exhaust on N/A Hemi trucks.

Our shop truck is running 1 7/8" headers with no cats, and no muffler or piping behind the Y-pipe. It doesn't get much more open than that, but when paired up with 4.10s and a 3400 stall converter, it still comes off the line hard with consistent mid 1.6 60' times.

Modifying a vehicle is a package deal, so you really want to have a goal in mind before starting any mods. I have learned this the hard way on previous vehicles and wasted a lot of money buying parts twice.

Yep, exhaust systems are defiantly application specific and you Must have a goal for what your trying to accomplish. :favorites13:

sicmadek · Mar 5, 2015

14hemiexpress said:
Couple things 1 it can slow down just depends on how hard you hold your thumb over it. Second that speed creates a vacume as a engine doesn't produce a solid stream of exhaust it's pulses and the first one can actually pull the second out so on and so forth "it's called scavenging" 3rd smaller isn't always better there is a point were too small will cause problems there is a sweet spot depending on what rpm your trying to produce power but it's normally smaller than most people think it will be.

Just saying the garden hose isnt the best analogy. Fluid vs gas apples vs oranges

hemihustlin · Mar 5, 2015

And that at high rpms the scavenging effect is reduced because the pulses are coming so fast its almost like a constant flow.

Pulled from the second link

This (scavenging) has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

14hemiexpress · Mar 5, 2015

sicmadek said:
Just saying the garden hose isnt the best analogy. Fluid vs gas apples vs oranges

There is truth to that but it's something you can see that's why I like it.

14hemiexpress · Mar 5, 2015

hemihustlin said:
And that at high rpms the scavenging effect is reduced because the pulses are coming so fast its almost like a constant flow.

Pulled from the second link

That's why long tubes normally lose on the low end but pick up on the top end.

Exhaust Systems 101

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